1. Field of the Invention
The present invention relates to an interdigital bandpass filter for use in a microwave band or a sub-microwave band.
2. Description of the Prior Art
An interdigital bandpass filter (hereinafter abbreviated as "the IBP filter") is, as is well-known, structured such that a plurality of resonator elements are arranged at selected intervals. Each of the resonator elements comprises a quarter wavelength line which has one end open-circuited and the other end short-circuited. Conventional IBP filters of this kind may be generally classified into three types. The first type is a cylindrical rod (or square rod) IBP filter, an example of which is described in "Microwave Circuit" by Munenori Ishii, Seizou Azuma, Toshlo Aoki and Kunlo Oi, pp.122-125, published on Feb. 28, 1969 by Nikkan Kogyo Simbun-sha. This first type of IBP filter comprises resonators constituted of cylindrical metal rods of different diameters which are mounted in a metal case (the case is grounded) such that the resonators alternately extend from the opposite sides of the inner surfaces of the metal case within the space defined in the metal case.
The second type of IBP filter is composed of a pair of comb-like resonator arrays, each comprising a plurality of integrated resonator elements in parallel with each other, wherein the combs are interdigitated. An example of this type is described in "Microwave Filters, Impedance-Matching Networks, and Coupling Structures", by George L. Matthaei, Leo Young and E.M.T. Jones, pp.621-631, published by McGraw-Hill Book Company in 1964.
The third type of IBP filter employs microstrip lines. An example of the third type is described in the article "Interdigital Microstrip Circuit Parameters Using Empirical Formulas and Simplified Model", by Arne Brejning Dalby, pp.744-752, IEEE Transactions On Microwave Theory and Techniques Vol. MTT-27, No. 8, August, 1979. This IBP filter is formed by employing, for example, a dielectric substrate which has both surfaces covered with copper foil, patterning one of the copper layers, and leaving a plurality of strip lines in parallel with each other (each line constitutes a resonator element).
In the conventional IBP filters as described above, the first and second types generally present good performance (for example, in terms of loss in a pass band, flatness of the characteristics in the band, attenuation characteristics on both sides of the pass band), but are not appropriate for mass production, while the third type presents good mass-productivity but is inferior in performance.
More specifically, the first type of IBP filter requires, for precisely mounting in a metal case resonators of different diameters with selected different intervals therebetween, fine machining of the respective resonator elements (lathe machining), solid machining (cutting, casting and so on) of the metal case, fine machining of the fixing means for mounting the resonator elements in the metal case (for example, threading of the resonator elements, drilling of screw holes into the metal case, etc.). Since the filter uses a number of parts and is assembled by a number of processes, it is not appropriate for mass production. Also, since the filter is structured such that individual resonator elements are separately mounted in a metal case, it is susceptible to the influence of mechanical vibrations.
The second type of IBP filter must precisely arrange a pair of resonator array combs relative to each other, and therefore generally requires fine adjustment of the relative distance of the combs. For this reason, like the first type, it is not appropriate for mass production.
Although it is relatively easy to form resonator elements to precise sizes and arrange them relative to each other in the third type of IBP filter, this filter employs a dielectric (not air) substrate which results in greater high frequency loss. Further, since it is not provided with a case for covering strip lines, electromagnetic radiation occurs. This filter, therefore, has the problems that Q is low and the selection characteristic is bad as compared with the foregoing two types.